An organic electroluminescent (OEL) device, alternately known as organic light-emitting diode (OLED), is useful in flat-panel display applications. This light-emissive device is attractive because it can be designed to produce red, green, and blue colors with high luminance efficiency, operable with a low driving voltage of the order of a few volts and viewable from oblique angles. These unique attributes are derived from a basic OLED structure comprising a multilayer stack of thin films of small-molecule organic materials sandwiched between an anode and a cathode. Tang et al. in commonly assigned U.S. Pat. Nos. 4,769,292 and 4,885,211 have disclosed such a structure. The common electroluminescent (EL) medium is comprised of a bilayer structure of a hole-transport layer (HTL) and an electron-transport layer (ETL), typically on the order of a few tens of nanometer (nm) thick for each layer. The anode material is typically optically transparent and the cathode is typically a metallic thin film. When an electrical potential difference is applied at the electrodes, the injected carriers (hole at the anode and electron at the cathode) migrate towards each other through the EL medium, and a fraction of them recombines in the emitting layer (EML) in a region close to the HTL/ETL interface, to emit light. The intensity of electroluminescence is dependent on the EL medium, drive voltage, and charge injecting nature of the electrodes. However, a significant fraction of generated light is trapped within the device in waveguiding modes and only a small fraction emerges out of the device. The light generated within the device emits in all directions. The light emitted toward the anode at less than the critical angle passes through the anode and substrate to the viewer, and the light emitted in the opposite direction is reflected at the cathode and passes through the substrate, enhancing the viewing intensity. A high-transparency substrate and anode and a high-reflectivity cathode are thus preferred.
Selection of electrode materials is also based on work functions. Indium tin oxide (ITO) is most commonly used as the anode because it is highly transparent and conductive and because it has a high work function. Mg:Ag alloys are generally used as electron-injecting contacts because they have lower work functions and highly reflective. Lithium containing reflective alloys such as Al:Li or Al on LiF also provides efficient electron injection to the EL medium.
The substrate generally used for OLEDS is glass. The ITO anode layer is most commonly sputtered on the glass substrate held at high temperature. The EL medium comprising organic layers are fabricated on the ITO glass substrates using a high vacuum deposition method. The cathode is fabricated by depositing a metallic layer over the organic medium also using the high vacuum method. Among the deposition methods for the cathode layer, resistive heating or electron-beam heating has been found to be most suitable because they do not cause damage to the organic layers. However, it would be highly desirable to avoid these methods for cathode fabrication. This is because these processes are inefficient. In order to realize low cost manufacturing, a robust and proven high-throughput process should be adopted and developed specifically for the OLEDS. Sputtering has been used as a method of choice for thin film deposition in many industries. Conformal, dense and adherent coatings, short cycle time, low maintenance of coating chamber, and efficient use of materials are some of the benefits of sputtering.
Sputtering is not commonly practiced for fabrication of the OLED cathodes because of the damage inflicted on the organic layers and the degradation of device performance. Sputter deposition takes place in a high energy and complex environment that is comprised of energetic neutrals, electrons, positive and negative ions, and emissions from the excited states that can degrade the organic layers upon which the cathode is deposited.
Liao et al. in Appl. Phys. Lett. 75, 1619 (1999) investigate, using x-ray and ultraviolet photoelectron spectroscopies, the damages induced on Alq surfaces by 100 eV Ar+irradiation. The core level electron density curves show that some N—Al and C—O—Al bonds in Alq molecules were broken. The valance band structure was also tremendously changed implying the formation of a metal-like conducting surface. The reference suggests that this would cause nonradiative quenching in OLEDS when electrons are injected into the Alq layer from the cathode and also would result in electrical shorts.
During sputter deposition of the cathode, the Alq surface is subjected to high doses of Ar+ bombardments at several hundred volts. As shown by Hung et al. in J. Appl. Phys. 86, 4607 (1999), a dose only of 9×1014/cm2 alters the valance band structure. Therefore, sputtering a cathode on Alq surface in Ar atmosphere is expected to degrade the device performance.
Sputtering damage is controllable, to some extent, by properly selecting the deposition parameters. In EP 0 876 086 A2, EP 0 880 305 A1, and EP 0 880 307 A2, Nakaya et al. of TDK Corporation disclosed a method of depositing a cathode by a sputtering technique. After depositing the organic layers, with vacuum still kept, the devices were transferred from the evaporation to a sputtering chamber wherein the cathode layer was deposited directly on the electron-transport layer. The cathode was an Al alloy comprised of 0.1-20% Li that additionally contained at least one of Cu, Mg, and Zr in small amounts, and in some cases had a protective overcoat. The OLED devices thus prepared used no buffer layer. Grothe et al. in DE 198 07 370 C1 also disclose a sputtered cathode of an Al:Li alloy, which had relatively high Li concentration and having one or more additional elements chosen from Mn, Pb, Pd, Si, Sn, Zn, Zr, Cu, and SiC. In all of the examples no buffer was used, yet electroluminescent was produced at lower voltages. Sputtering damage was controlled by employing a low deposition rate. By lowering sputtering power it is believed that damage inflicted on the organic layers can be reduced. At low power, however, the deposition rate can be impracticably low and the advantages of sputtering are reduced or even neutralized.
To reduce damage during high speed sputtering of cathodes, a protective coating on the electron-transport layer can be useful. The protective layer, alternately termed as the buffer layer, should be robust to be effective. Parthasarathy et al. in Appl. Phys. Lett. 72, 2138 (1998) report an application of a buffer layer including copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) during sputtering deposition of a metal free cathode. The buffer layer prevented damage to the underlying organic layers during the sputtering process. Hung et al. in J. Appl. Phys. 86, 4607 (1999) disclose the application of CuPc buffer layers that permit high-energy deposition of alloy cathodes. The cathode contained a low work function component, Li, which was believed to diffuse through the buffer layer and provided an electron-injecting layer between the electron-transport layer and the buffer layer. Nakaya et al. in EP 0 982 783 A2 disclose a cathode of Al:Li alloy. The cathode was prepared by sputtering using a buffer layer constructed of a porphyrinic or naphthacene compound that was deposed between the electron-transport layer and the cathode. The device containing the sputtered electrode exhibited low drive voltage, high efficiency, and retarded dark spot growth.
Raychaudhuri et al. disclose in U.S. Pat. No. 6,579,629 a high speed sputtering process for fabrication of cathodes. On the ETL was deposited by evaporation a buffer structure comprising two layers, one buffer layer containing an alkali metal halide, and a second buffer layer containing a metal phthalocyanine. An alloy cathode such as Ag or Al, containing an alkali metal such as Li, was deposited on the buffer structure at rates up to 22 A/s. The sputtered cathode devices were identical in performance to a control device having an evaporated cathode. It was thus concluded that the buffer structure provided complete protection from sputter damage.
Although efficient devices based on sputtered cathode were disclosed, the use of a metal phthalocyanine, e.g. CuPc, and an alloy target appear undesirable. The CuPc in the device structure absorbs in the yellow/red wavelength range, and can exhibit color shifts. It will be highly desirable to replace CuPc with a transparent buffer. The cathode was fabricated using an alloy target that contains an alkali metal e.g., Li. With this approach the target itself is the source of the electron-injecting dopant. Due to dissimilar properties, particularly with respect to melting point, vapor pressure and other properties of alkali metals and other component metals, fabrication of homogeneous and quality target can be quite difficult. It is desirable to use pure metal, as high quality targets are readily available.
P. K. Raychaudhuri and J. K. Madathil, “Fabrication of Sputtered Cathodes for Organic Light-Emitting Diodes Using Transparent Buffer”, Proceedings of the 7th Asian Symposium on Information Display (Sep. 2-4, Singapore) Digest, paper 50, Vol. 32, pp. 55-58, 2002 report sputtered cathode devices using transparent buffer, only several nanometers thick, and pure metal targets. The buffer layer is comprised of a fluoride of a heavy alkali metal upon which a metal such as Al or Mg was sputtered. These metals are reactive to alkali metal fluoride, and liberate an electron-injecting dopant by decomposing the fluoride. High efficiency devices, indistinguishable from evaporated cathode devices, were made by depositing the cathode by sputtering. However, in this case the thickness of buffer layer is to be carefully optimized so as not to leave excess unreacted buffer and to reduce build up of insulating reaction products at the ETL/cathode interface. In addition, the handling of alkali metal fluorides is difficult owing to their hygroscopic nature. Furthermore, the cathode metals are believed limited to those, which are reactive to the buffer layer.
A buffer layer of appropriate properties appears necessary in the device structure in a high speed sputtering process for the OLED cathode. A buffer layer should be robust and resistant to plasma, and should be able to protect the underlying organic layers from plasma damage. In addition, the buffer layer should permit the charge transport from the cathode to the ETL. If evaporable using low energy deposition process, metallic layers seem appropriate as a buffer. Since vacuum deposited Al is an electrode in the widely used LiF/Al cathode, the Al layer itself can be considered as a buffer layer upon which an additional layer of a protective metal or an alloy can be sputtered. This additional layer can be termed as a protective cathode layer or simply a protective layer. Al being reasonably resistant to atmospheric condition can also be useful as a protective cathode material. It has been found however, that even a moderately thick Al layer of the LiF/Al cathode fails to provide protection to the EL medium from sputter damage during sputtering of the Al protective cathode. Moreover, the sputtering process, which uses materials efficiently, and yields dense, conformal and adherent coatings with greatly reduced pinhole density, is desirable in large-scale manufacturing.